This invention relates to an optically clear thermoplastic resin composition, a method to synthesize the composition and articles made from the compositions.
Polycarbonate is a useful engineering plastic for parts requiring clarity, high toughness, and, in some cases, good heat resistance. However, polycarbonate also has some important deficiencies, among them poor chemical and stress crack resistance, poor resistance to sterilization by gamma radiation, and poor processability. Blends polyesters with polycarbonates provide thermoplastic compositions having improved properties over those based upon either of the single resins alone. Moreover, such blends are often more cost effective than polycarbonate alone.
Transparent blends of polycarbonate (PC) and polyesters have attractive properties such as toughness and chemical resistance. The miscibility of PC with the polyesters gives the blends the clarity needed, but this is restricted to (semi)aliphatic polyesters such as poly(cyclohexane dimethanol cyclohexane dicarboxylate) (PCCD) or a glycolized copolyester such as polyethylene glycol cyclohexane dimethanol terephthalate (PETG). PCT patent application no. WO 02/38675 discloses a thermoplastic composition comprising PC, PCCD, and an impact modifier.
U.S. Pat. Nos. 4,188,314, 4,125,572; 4,391,954; 4,786,692; 4,897,453, and 5,478,896 relate to blends of an aromatic polycarbonate and poly cyclohexane dimethanol phthalate. U.S. Pat. No. 4,125,572 relates to a blend of polycarbonate, polybutylene terephthalate (PBT) and an aliphatic/cycloaliphatic iso/terephthalate resin. U.S. Pat. No. 6,281,299 discloses a process for manufacturing transparent polyester/polycarbonate compositions, wherein the polyester is fed into the reactor after bisphenol A is polymerized to a polycarbonate.
The patents, U.S. Pat. Nos. 5,194,523 and 5,207,967 describe the blending of polyester (polycyclohexane dimethanol terephthalate, “PCT”) with bisphenol-A polycarbonate to obtain blends with improved low temperature impact strength and processability. The U.S. Pat. No. 4,786,692 describes a clear thermoplastic molding composition of polycarbonate with PETG with the ethylene glycol content between 20 to 80 percent. However the patent requires that the said lend becomes semi transparent and opaque when the ethylene glycol content is high at least greater than 50 percent. The U.S. Pat. No. 5,194,523 describes a clear blend of polycarbonate and PETG where the amount of ethylene glycol is between 15 percent and 30 percent. The blends were synthesized using the residual catalyst present in the polyester. U.S. Pat. No. 4,506,442 discloses a polycarbonate/polyester blend and an uncatalyzed process for preparing the blend by melt reactions between PC and polyesters for a long period of time (mixing time of up to 60 minutes).
The blends of polycarbonate with PETG having a ethylene glycol content of up to 67 percent prepared without the addition of external catalyst have been reported to be unstable. Polycarbonate blends with PETG wherein the ethylene glycol content is greater than 60 percent have been found to give opaque incompatible blends.
There is a continuing need for polycarbonate polyester blends having a good balance of transparency, processability, solvent resistance and environmental stress cracking resistance in addition to good mechanical and thermal properties.